1.6 3D grid building
Once the stratigraphic and the structural modeling done, the 3D grid can be built (Figure 1). The 3D grid is representing the volume of rocks inside each geological unit. The 3D grid is divided into cells, each cell representing a small piece of the reservoir. Typically, cells have size between 25m*25m and 100m*100m horizontally and 0.1m to 1m vertically. Each cell will contain a specific value for the different properties (facies + petrophysics).
Defining the orientation of the mesh of the 3D grid is an essential step of the modeling process. Most interpolation techniques will tend to populate the properties following the main directions of the mesh. Figure 3 illustrates this point with the vertical layering of the 3D grid.
Based on his interpretation of the data, the geologist decides that the horizon B is an unconformity. This leads the geomodeler to build the surface representing the horizon B following the first approach (Figure 2, see also section 1.4). Analysis of the core data shows that the reservoir is made of a massive fluvial sand channel surrounded by shale (Figure 3A). The sand is only visible on well 1. Regional data shows that it should extend further toward wells 2 and 3. How shall we interpolate the sand? If the vertical layering of the 3D grid is made horizontal (Figure 3B), the sand channel will be interpolated horizontally. If the vertical layering is made parallel to the horizon B (Figure 3C), the sand channel will dip like the horizon B does. If the vertical layering is made parallel to the horizon A (Figure 3D), the channel will have a more complex, curved geometry.
The choice again lies in the hands of the geologist. Similarly to the problem of the construction of the horizons, the answer isn’t found in the data alone (the core data here). The geomodeler also needs the involvement of the geologist. And as for the horizon building, in case the geologist has no way to be sure of which geometry should be built, the reservoir modeler might have to carry forward several models, one for each possible internal geometry of the 3D grid.
Over the last few years, a new set of techniques coupling the structural modeling, the stratigraphic modeling and the 3D grid building have gained popularity. Such integrated techniques simplify and improve the construction of the structural model, which has always been difficult for complex fault networks. These techniques also allow to align the mesh of the 3D grid to complex trends. For example, (Thenin and Larson, 2013) used these techniques to integrate the complex geometry of Inclined Heterolithic Strata (IHS) found in oil sand reservoirs into the mesh of the 3D grid. Such workflow can be extended to any reservoir in which seismic stratigraphy has been interpreted (Veeken and van Moerkerken, 2013). The details of the solutions implemented by the different software vendors are not yet all known. Implicit modeling seems to be at the core of at least some of those new tools. At this time, readers interested in the some of the mathematical details should refer to (Mallet, 2014) which just got published.